Photosynthesis in Poikilohydric Plants: A Comparison of Lichens and Bryophytes
327
found, even at 1500% water content (Snelgar et al. 1980). There seems little
doubt that bryophytes can deal with high thallus water content much better
than lichens. It must be noted that the complex Polytrichales, with ventilated
tissues, have a low maximum thallus water content at around 250%, much
more similar to thallus water content of lichens.
Although A declines at low thallus water content, it can continue at very
low water potentials. When lichens, bryophytes, and higher plants were
equilibrated on solutions of sucrose, mannitol, or sorbitol, the higher plants
(Nerium oleander and Spinacea oleracea) reached zero A at water potentials
of about -5 to -6 MPa. Bryophytes were similar or reached slightly
lower potentials (Kaiser 1987; Dilks and Proctor 1979). Lichens with green
(Chlorophycean) photobionts continued to photosynthesize below -20 MPa
and, in equilibrium with water vapor, could reach -38 MPa, in the case of
Dendrographa minor (Lange 1988). Moreover, these lichens showed similar
photosynthetic rates at low potentials whether equilibrated with an osmotic
solute or with atmospheric water vapor (Nash et al. 1990). This ability to
utilize atmospheric water vapor is confined to green algal lichens (Lange et
al. 1988). Interestingly, Kappen (1993) has suggested that the behavior of
lichens when frozen may be identical to that when they are in equilibrium
with water vapor. In both cases the photobiont cells are subjected to low
water potentials following concentration, by water loss, of the cell contents.
Cyanobacterial lichens cannot achieve positive net photosynthesis in equilibrium with water vapor but, in equilibrium with a solute potential, behave
similarly to higher plants. It seems possible, based on limited evidence, that
some bryophytes may also show identical photosynthetic behavior when in
equilibrium with water vapor or liquid water (Lange 1969b). Their ability to
use water vapor for reactivation of net photosynthesis after desiccation
seems to be much more restricted than with lichens (see Rundel and Lange
1980). The green lichens stand out quite distinctly and can exhibit considerable discrimination against 13C02, suggesting that some may carry out most
of their photosynthesis in the vapor-equilibrated state since liquid water
would lead to less discrimination (Lange et al. 1988).
16.2.6 Environmental CO2 Concentration
Lichens and bryophytes typically grow in close contact with their substrate.
When this substrate has an organic component, it is likely that there will be
CO2 evolution from respiratory processes in the substrate, such as in tundra,
peat mire, and forest ecosystems (Sveinbjornsson and Oechel 1992).
Because of this CO2 efflux, local CO2 levels around bryophytes and lichens
may be considerably higher than normal ambient levels (340- 350 j.lIC0 2 1- 1
air). This topic has recently been reviewed and good evidence for substantial
CO2 effluxes exists but there are only a few data on local CO 2 elevation
(Sveinbjornsson and Oechel 1992). Measurements from a New Zealand
327
found, even at 1500% water content (Snelgar et al. 1980). There seems little
doubt that bryophytes can deal with high thallus water content much better
than lichens. It must be noted that the complex Polytrichales, with ventilated
tissues, have a low maximum thallus water content at around 250%, much
more similar to thallus water content of lichens.
Although A declines at low thallus water content, it can continue at very
low water potentials. When lichens, bryophytes, and higher plants were
equilibrated on solutions of sucrose, mannitol, or sorbitol, the higher plants
(Nerium oleander and Spinacea oleracea) reached zero A at water potentials
of about -5 to -6 MPa. Bryophytes were similar or reached slightly
lower potentials (Kaiser 1987; Dilks and Proctor 1979). Lichens with green
(Chlorophycean) photobionts continued to photosynthesize below -20 MPa
and, in equilibrium with water vapor, could reach -38 MPa, in the case of
Dendrographa minor (Lange 1988). Moreover, these lichens showed similar
photosynthetic rates at low potentials whether equilibrated with an osmotic
solute or with atmospheric water vapor (Nash et al. 1990). This ability to
utilize atmospheric water vapor is confined to green algal lichens (Lange et
al. 1988). Interestingly, Kappen (1993) has suggested that the behavior of
lichens when frozen may be identical to that when they are in equilibrium
with water vapor. In both cases the photobiont cells are subjected to low
water potentials following concentration, by water loss, of the cell contents.
Cyanobacterial lichens cannot achieve positive net photosynthesis in equilibrium with water vapor but, in equilibrium with a solute potential, behave
similarly to higher plants. It seems possible, based on limited evidence, that
some bryophytes may also show identical photosynthetic behavior when in
equilibrium with water vapor or liquid water (Lange 1969b). Their ability to
use water vapor for reactivation of net photosynthesis after desiccation
seems to be much more restricted than with lichens (see Rundel and Lange
1980). The green lichens stand out quite distinctly and can exhibit considerable discrimination against 13C02, suggesting that some may carry out most
of their photosynthesis in the vapor-equilibrated state since liquid water
would lead to less discrimination (Lange et al. 1988).
16.2.6 Environmental CO2 Concentration
Lichens and bryophytes typically grow in close contact with their substrate.
When this substrate has an organic component, it is likely that there will be
CO2 evolution from respiratory processes in the substrate, such as in tundra,
peat mire, and forest ecosystems (Sveinbjornsson and Oechel 1992).
Because of this CO2 efflux, local CO2 levels around bryophytes and lichens
may be considerably higher than normal ambient levels (340- 350 j.lIC0 2 1- 1
air). This topic has recently been reviewed and good evidence for substantial
CO2 effluxes exists but there are only a few data on local CO 2 elevation
(Sveinbjornsson and Oechel 1992). Measurements from a New Zealand